KR102020434B1 - Steel material having exellent hydrogen induced crack resistance and low temperature impact toughness and method of manufacturing the same - Google Patents

Abstract

The present invention is in weight%, C: 0.02 to 0.12%, Si: 0.02 to 0.60%, Mn: 0.6 to 1.6%, P: 0.020% or less (excluding 0%), S: 0.003% or less (excluding 0%) ), Sol.Al: 0.002 to 0.060%, Cu: 0.005 to 0.6%, Ni: 0.005 to 1.0%, Cr: 0.005 to 0.5%, Mo: 0.01 to 0.3%, Ti: 0.001 to 0.10%, Nb: 0.001 to 0.06%, V: 0.001-0.10%, N: 0.001-0.006%, Ca: 0.0002-0.0060%, balance Fe and other unavoidable impurities, (Ti + V + 0.5Nb + 0.5Mo) is 0.05% or more , (Ti + V + 0.5 Nb + 0.5 Mo) / C is 0.8 or more, ferrite as a microstructure; and one or more of the remaining pearlite, cementite and MA (martensite-austenite composite phase), the ferrite The fraction of is 90% or more (excluding 100%) in area%, and the total fraction of at least one of the pearlite, cementite and MA (martensite-austenite composite phase) is 10% or less in area%, Fine carbonitride precipitates with an average size of 100 nm or less are present in an amount of 0.03% or more by weight, Sites provides a high-strength steel material has an average crystal grain size is not more than 20㎛ hydrogen induced crack resistance and low temperature impact toughness, excellent and a method of manufacturing the tissue.

Description

High strength steel with excellent hydrogen organic crack resistance and low temperature impact toughness and its manufacturing method {STEEL MATERIAL HAVING EXELLENT HYDROGEN INDUCED CRACK RESISTANCE AND LOW TEMPERATURE IMPACT TOUGHNESS AND METHOD OF MANUFACTURING THE SAME}

The present invention relates to a high-strength steel used in a plant facility and a shipbuilding and offshore structure and the like for mining, processing, transporting, and storing crude oil, and more specifically, hydrogen organic crack resistance (HIC characteristics) and low temperature impact toughness. It relates to an excellent high strength steel and a method of manufacturing the same.

Due to the depletion of petroleum resources and high oil prices, the production of low-quality crude oil with high impurity content such as sulfur is gradually increasing. As a result, steel materials used in all plant facilities for mining, processing, transporting and storing low-grade crude oil are also crude oil. It is indispensable to have a property of inhibiting hydrogen induced cracking (HIC) due to wet hydrogen sulfide by the sulfur component of the genus.

In addition, environmental pollution caused by recent safety accidents are a global problem, and as astronomical costs are required to recover them, the level of the required characteristics of steel materials used in the energy industry is gradually increasing.

High-strength normalizing steels have been proposed as an example of materials for plant equipment for mining, processing, transporting, storing crude oil, and shipbuilding and offshore structures.

As a method for obtaining high strength in normalizing steels, an increase in the perlite fraction by addition of C or addition of solid solution strengthening elements such as Cu and Mo, and production of carbonitride precipitates by simple addition of Nb and V are utilized.

On the other hand, as a method for improving the resistance to hydrogen organic cracking of the steel, a method of controlling internal defects such as inclusions and voids in the steel, which may act as a starting point of hydrogen accumulation and cracking, and cracks are easily generated and propagated. In order to minimize hardened structures, such as pearlite, or to control the shape, the method of applying strong rolling etc. is widely proposed and used.

However, as the required strength of steel materials gradually increases, the amount of C added or the amount of solid solution strengthening component is increased to secure strength. However, an increase in the amount of C added increases the perlite fraction and also makes the perlite appear in a banded streucture, which not only facilitates the propagation of hydrogen organic cracks, but also causes a significant decrease in impact toughness at low temperatures. Bumped into

In addition, the addition of a solid solution strengthening component such as Cu, Ni, Mo, but has the effect of increasing the strength, causing the surface cracks due to redness brittleness, or cause a large reduction in economic efficiency due to the high price.

In addition, when an excessive amount of Nb and V is added to obtain the strengthening effect of the precipitate, the hardenability of the weld part is greatly increased beyond the reference level, thereby acting as a cause of deterioration of the toughness of the weld part and hydrogen organic cracking.

As described above, the conventional method has a problem in that it is difficult to simultaneously secure high strength of the normalizing steel, excellent impact toughness at low temperature, and excellent hydrogen organic crack prevention characteristics.

Therefore, there is a demand for development of a normalizing steel having both high strength and excellent impact toughness at low temperatures and excellent hydrogen organic crack prevention characteristics.

Republic of Korea Patent Publication No. 2011-0060449

One preferred aspect of the present invention is to provide a high strength steel having excellent hydrogen organic crack resistance (HIC resistance) and low temperature impact toughness.

Another preferred aspect of the present invention is to provide a method for producing a high strength steel excellent in hydrogen organic crack resistance (HIC resistance) and low temperature impact toughness.

According to a preferred aspect of the present invention, in weight%, C: 0.02 to 0.12%, Si: 0.02 to 0.60%, Mn: 0.6 to 1.6%, P: 0.020% or less (excluding 0%), S: 0.003% (Excluding 0%), Sol.Al: 0.002 to 0.060%, Cu: 0.005 to 0.6%, Ni: 0.005 to 1.0%, Cr: 0.005 to 0.5%, Mo: 0.01 to 0.3%, Ti: 0.001 to 0.10 %, Nb: 0.001-0.06%, V: 0.001-0.10%, N: 0.001-0.006%, Ca: 0.0002-0.0060%, balance Fe and other unavoidable impurities, (Ti + V + 0.5Nb + 0.5Mo ) Is 0.05% or more, (Ti + V + 0.5Nb + 0.5Mo) / C is 0.8 or more, ferrite as a microstructure; and at least one of the remaining pearlite, cementite and MA (martensite-austenite composite phase) Wherein the fraction of ferrite is 90% or more (excluding 100%) as area%, and the total fraction of at least one of the pearlite, cementite and MA (martensite-austenite composite phase) is 10% or less as area%. In the ferrite structure, fine carbonitride precipitates having an average size of 100 nm or less are 0.03% by weight. The high strength steel which exists above and is excellent in hydrogen organic crack resistance (HIC characteristic) and low temperature impact toughness whose average grain size of the said ferrite structure is 20 micrometers or less is provided .

The steel is manufactured by a steel manufacturing method including a normalizing heat treatment step, and the fraction of ferrite, pearlite, cementite and MA in the microstructure after the normalizing heat treatment of the steel increases the ferrite by 10% or less compared to before the normalizing heat treatment. Perlite may decrease by less than 10%, cementite may increase by 5%, and MA may decrease by 3%.

According to another preferred aspect of the present invention, in weight%, C: 0.02 to 0.12%, Si: 0.02 to 0.60%, Mn: 0.6 to 1.6%, P: 0.020% or less (excluding 0%), S: 0.003 % Or less (excluding 0%), Sol.Al: 0.002 to 0.060%, Cu: 0.005 to 0.6%, Ni: 0.005 to 1.0%, Cr: 0.005 to 0.5%, Mo: 0.01 to 0.3%, Ti: 0.001 to 0.10%, Nb: 0.001-0.06%, V: 0.001-0.10%, N: 0.001-0.006%, Ca: 0.0002-0.0060%, balance Fe and other unavoidable impurities, (Ti + V + 0.5Nb + 0.5 Reheating the slab having Mo) of 0.05% or more and (Ti + V + 0.5Nb + 0.5Mo) / C of 0.8 or more to 1080-1300 ° C .;

Obtaining a hot rolled steel by controlling and rolling the slab reheated as described above under a condition that a rolling end temperature is 850 ° C. or more;

A controlled cooling step of cooling the hot rolled steel to a cooling end temperature of 550 to 750 ° C. and then air cooling the hot rolled steel; And

Normalized heat treatment step of maintaining the cooled steel as described above 1.3 * t [t is the steel sheet thickness (mm)] + (5 ~ 60 minutes) at 850 ~ 950 ℃, then air-cooled; Provided is a method for producing a high strength steel having excellent hydrogen organic crack resistance (HIC resistance) and low temperature impact toughness .

The method may further include tempering heat treatment of the heat-treated steel according to the normalizing heat treatment step at 500 ° C. or more for 10 minutes or more.

According to a preferred aspect of the present invention, it is possible to provide a high strength steel having excellent hydrogen organic crack resistance and low temperature impact toughness that can be effectively applied to plant equipment, shipbuilding and offshore structures and pressure vessels for mining, processing, transporting and storing crude oil. .

1 is a microstructure photograph of a steel material (Comparative Example 3) prepared according to the conventional method.
Figure 2 is a microstructure picture of the steel (invention example 1) prepared according to the present invention.
Figure 3 is a photograph showing the distribution of precipitates in the steel (Comparative Example 3) prepared according to the conventional method.
Figure 4 is a photograph showing the precipitate distribution of the steel material (invention example 1) prepared according to the present invention.

Hereinafter, preferred embodiments of the present invention will be described.

However, embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

In addition, embodiment of this invention can be modified in various other forms, The range of this invention is not limited to embodiment described below.

In addition, the inclusion of any component throughout the specification means that it may further include other components, except to exclude other components unless specifically stated otherwise.

One of the main concepts of the present invention is to maximize the fraction of fine ferrite in order to secure both HIC propagation resistance and strength, and to have a microstructure composed of the hard phases such as cementite, pearlite and MA phases, which are disadvantageous microstructures. In order to produce a nano-sized fine precipitate in the ferrite grains by hot rolling (control rolling) and water cooling (control cooling) to greatly suppress the growth after the formation of austenite during the subsequent heat treatment of the ferrite produced during cooling By controlling the grain size finely, it is intended to simultaneously improve hydrogen organic crack resistance (HIC resistance), low temperature impact toughness and strength by fine ferrite and precipitates.

The present invention is to control the content of C in the range of 0.12% or less so as to lower the content of C than the conventional method, in order to produce a high-strength normalizing steel having excellent hydrogen organic crack resistance and low temperature toughness, the microstructure of the ferrite 90% or more However, by controlling the cooling conditions after hot rolling, the presence of 0.03% by weight or more of fine precipitates of 100 nm or less in the ferrite grains suppresses the growth after the formation of austenite during normalizing heat treatment, and thus the ferrite grains generated during cooling are reduced. At the same time, the total fraction of at least one of the pearlite, cementite, and MA phases, which are disadvantageous to low-temperature toughness and hydrogen organic cracking prevention properties, is controlled to be 10% or less, and a large amount of fine precipitates is used in addition to the effect of increasing strength. It also acts as a fixation site for absorbed hydrogen, resulting in hydrogen organic cracking The stars (in the HIC characteristic) and the low-temperature impact toughness to provide an excellent high-strength steel and a method of manufacturing the same.

Hereinafter, a high strength steel having excellent hydrogen organic crack resistance (HIC resistance) and low temperature impact toughness according to a preferred aspect of the present invention will be described in detail.

High-strength steel having excellent hydrogen organic crack resistance (HIC resistance) and low temperature impact toughness according to a preferred aspect of the present invention is a weight%, C: 0.02 to 0.12%, Si: 0.02 to 0.60%, Mn: 0.6 to 1.6% , P: 0.020% or less (excluding 0%), S: 0.003% or less (excluding 0%), Sol.Al: 0.002 to 0.060%, Cu: 0.005 to 0.6%, Ni: 0.005 to 1.0%, Cr: 0.005 to 0.5%, Mo: 0.01 to 0.3%, Ti: 0.001 to 0.10%, Nb: 0.001 to 0.06%, V: 0.001 to 0.10%, N: 0.001 to 0.006%, Ca: 0.0002 to 0.0060%, balance Fe and Contains other unavoidable impurities, (Ti + V + 0.5Nb + 0.5Mo) is 0.05% or more, (Ti + V + 0.5Nb + 0.5Mo) / C is 0.8 or more, ferrite into the microstructure; and the remaining pearlite , At least one of cementite and MA (martensite-austenite composite phase), wherein the fraction of ferrite is 90% or more (excluding 100%) in area%, and the pearlite, cementite and MA (martensite-austen) Knight composite phase), the total fraction of one or more species is 10% or less by area%, Fine carbonitride precipitates having an average size of 100 nm or less are present in the light tissue by 0.03% or more by weight, and the average grain size of the ferrite tissue is 20 µm or less.

Hereinafter, the component and the component range will be described.

C: 0.02 to 0.12% by weight (hereinafter also referred to as "%")

In normalizing steels, C is generally added to form a pearlite, cementite or MA phase to secure tensile strength, but in the present invention, it is added as an important element to produce fine precipitates of 100 nm or less, the content of which is 0.02 It is preferable to limit it to -0.012%. If the content of C is less than 0.02%, the fraction of carbonitride precipitates and the fraction of pearlite may be lowered, and thus the tensile strength on the matrix may be lowered. If the content of C is more than 0.12%, a hard phase such as excessive pearlite is formed and subsequent rolling is performed. The presence of a band at can reduce the impact toughness at low temperatures as well as the hydrogen organic crack resistance. The more preferable content of C is 0.03 to 0.10%, and even more preferred content of C is 0.05 to 0.08%.

Si: 0.02 ~ 0.60%

Si is an element added for the purpose of solid solution strengthening with deoxidation and desulfurization effect, and it is preferable to add Si or more to secure yield strength and tensile strength.

On the other hand, if the content exceeds 0.60%, weldability and low temperature impact properties are lowered, and the surface of the manufactured steel sheet is easily oxidized to form an oxide film severely, so the content of Si is preferably limited to 0.02 to 0.60%. More preferable Si content is 0.05-0.45%, and even more preferable Si content is 0.05-0.35%.

Mn: 0.6 ~ 1.6%

Mn is an element having a large effect of increasing strength due to solid solution strengthening, and it is preferable to add Mn at least 0.6%. On the other hand, when Mn is added excessively, segregation becomes severe in the thickness direction center part of a steel plate, and it promotes formation of MnS which is a nonmetallic inclusion with S segregated at the same time. The MnS inclusions generated in the center portion are drawn by subsequent rolling, and as a result, the low-temperature toughness and hydrogen organic crack resistance are greatly reduced, so the content of Mn is preferably limited to 1.6% or less. More preferred Mn content is 0.8-1.45%, and even more preferred Mn content is 1.1-1.3%.

P: 0.020% or less (except 0%)

P has the effect of increasing the strength when added, but in contrast, in the heat-treated steel, it is preferable to manage it as low as possible because it is an element that greatly deteriorates low-temperature toughness due to grain boundary segregation. However, it is preferable to limit it to 0.020% or less (except for 0%) because it is expensive to remove it excessively in the steelmaking process.

S: 0.003% or less (except 0%)

S combines with Mn to form MnS inclusions mainly in the thickness direction center of the steel sheet, which not only degrades low-temperature toughness but is also a representative factor for promoting the generation and propagation of hydrogen organic cracks. Therefore, in order to secure hydrogen organic cracking resistance, S should be removed as much as possible in the steelmaking process, but it is preferable to limit the amount to 0.003% or less since excessive cost may be required. More preferable S content is 0.001% or less.

Sol.Al: 0.002 ~ 0.060%

Al is added as a strong deoxidizer in the steelmaking process together with Si and Mn, and at least 0.002% at the time of single or complex deoxidation can be obtained to obtain the maximum effect. On the other hand, the addition of more than 0.060%, the effect is saturated, the fraction of Al ₂ O ₃ in the oxidative inclusions produced as a result of deoxidation increases more than necessary, the size is coarse, may not be removed well during refining. Thus, Sol. The content of Al is preferably limited to 0.002 to 0.06%. More preferred Sol. The content of Al is 0.005 to 0.04%, more preferred Sol. The content of Al is 0.01 to 0.025%.

Cu: 0.005 ~ 0.6%

Cu is a component which can improve a strength significantly by solid solution and precipitation, and has an effect which suppresses corrosion in a wet hydrogen sulfide atmosphere, and is added. Excessive Cu addition causes cracks on the surface of the steel sheet, and it is preferable to limit the Cu content to 0.005 to 0.6% in view of economical efficiency as an expensive element . The more preferable content of Cu is 0.02 to 0.4%, and even more preferable content of Cu is 0.05 to 0.3%.

Ni: 0.005-1.0%

Ni has little effect of increasing strength, but is effective in improving low-temperature toughness, and particularly has an effect of suppressing surface cracks due to selective oxidation generated when the slab is reheated when Cu is added. Ni is added to obtain such an effect, and Ni is also an expensive element, and from the viewpoint of economics, the content is preferably limited to 0.005 to 1.0%. The more preferable content of Ni is 0.02 to 0.5%, and the more preferable content of Ni is 0.05 to 0.3%.

Cr: 0.005-0.5%

Cr has a small effect of increasing yield strength and tensile strength by solid solution, but is added at least 0.005% because it has an effect of slowing down the decomposition rate of cementite during tempering or post-weld heat treatment. On the other hand, when Cr is added in excess of 0.5%, it is preferable to limit the content of Cr to 0.5% or less since it not only increases costs but also deteriorates weldability. The more preferable content of Cr is 0.01 to 0.4%, and the more preferable content of Cr is 0.02 to 0.2%.

Mo: 0.01 ~ 0.3%

Mo is an element that is generally effective in preventing strength drop during tempering or heat treatment after welding, such as Cr, and has an effect of preventing toughness deterioration due to grain boundary segregation of impurities such as P. In the present invention, Mo has the effect of greatly increasing the strength by generating nano-sized carbonitride.

On the other hand, Mo is an expensive element and when added in excess of 0.3%, the hardenability is excessively increased to promote a hardened phase that degrades toughness and hydrogen organic crack resistance, so the content thereof is preferably limited to 0.01 to 0.3%. Do. More preferable Mo content is 0.02 to 0.2%, and still more preferable Mo content is 0.05 to 0.15%.

Ti: 0.001-0.10%

As Ti is added, nano-size carbonitrides are greatly increased, but when added at more than 0.001%, when added at more than 0.10%, it is present as coarse precipitate of hexagonal TiN form and impact test at low temperature. As well as at the onset of hydrogen organic cracking. Therefore, the content of Ti is preferably limited to 0.10% or less. The more preferable content of Ti is 0.002 to 0.05%, and even more preferred content of Ti is 0.005 to 0.03%.

Nb: 0.001-0.06%

Nb is dissolved in austenite during slab reheating to increase the hardenability of austenite, and precipitates as carbonitride that matches with the matrix at high temperatures during hot rolling to suppress recrystallization and thus make the final microstructure fine. In addition, there is an effect of greatly increasing the strength by generating a fine precipitate having a size of less than 100nm even during transformation after cooling.

However, when Nb is added in an excessively large amount, not only is it easy to form coarse precipitates in the central portion in the thickness direction, but also increases the hardenability of the weld portion more than necessary, thereby lowering the low temperature toughness and hydrogen organic crack resistance characteristics, so that the content of Nb is 0.001. It is preferable to limit it to -0.06%. The more preferable content of Nb is 0.002 to 0.05%, and the more preferable content of Nb is 0.005 to 0.04%.

V: 0.001 to 0.10%

When V reheats the slab, almost all of them are re-used, and there is little reinforcement effect due to precipitation or solid solution in the rolled state. However, V is precipitated as very fine carbonitride during tempering or post-weld heat treatment to improve strength. However, due to the high price, the content of V is preferably limited to 0.001 to 0.10%. The more preferable content of V is 0.001 to 0.06%, and even more preferred content of V is 0.005 to 0.02%.

N: 0.001-0.006%

N forms precipitates with added Nb, Ti, Mo, and V to refine the grains of the steel to improve the strength and toughness of the base metal, but when excessively added, it exists in an excess of atomic state to promote embrittlement at high temperatures. Can be. In addition, during the reheating of the slab, the restocking fraction of carbonitrides is reduced, which causes a significant reduction in the amount of fine precipitates, which is an important means of the present invention. Therefore, the content of N is preferably limited to 0.001 ~ 0.006% in consideration of the content of Nb, V, Mo and Ti and the slab reheating temperature. The more preferable content of N is 0.002 to 0.005%, and even more preferred content of N is 0.002 to 0.0045%.

Ca: 0.0002-0.0060%

When Ca is added after Al deoxidation, it combines with S present as MnS to suppress MnS formation and to form spherical CaS to suppress hydrogen organic cracking. Therefore, it is preferable to add Ca or more than 0.0002% in order to sufficiently form the added S to CaS. On the other hand, when the amount of Ca added is excessive, excess Ca combines with O (oxygen) to produce coarse oxidative inclusions, which are stretched and fractured in subsequent rolling to promote hydrogen organic cracking. Therefore, the upper limit of the Ca content is preferably limited to 0.0060%. The more preferable content of Ca is 0.0005 to 0.0060%, and the more preferable content of Ca is 0.001 to 0.0025%.

(Ti + V + 0.5Nb + 0.5Mo): 0.05% or more, (Ti + V + 0.5Nb + 0.5Mo) /C:0.8 or more

 The reason for limiting the content of Ti, Nb, V, and Mo, which generate denitrification precipitates, is to generate enough nano-sized precipitates during normalizing heat treatment to suppress the growth of austenite and thus to reduce ferrite grains. In addition to miniaturization, there is an effect of greatly increasing the strength due to the fine precipitates themselves, so the content of these elements should be added above a certain level.

 In addition, fine precipitates can be sufficiently produced only when the ratio with C, which is an important constituent of carbonitride, is high.

Therefore, it is preferable to limit (Ti + V + 0.5Nb + 0.5Mo) to 0.05% or more and (Ti + V + 0.5Nb + 0.5Mo) / C to 0.8 or more.

The high strength steel having excellent hydrogen organic crack resistance (HIC resistance) and low temperature impact toughness according to a preferred aspect of the present invention is ferrite as a microstructure; and one of the remaining pearlite, cementite and MA (martensite-austenite composite phase). Including the above, the fraction of the ferrite is 90% or more (excluding 100%) in area%, the total fraction of at least one of the pearlite, cementite and MA (martensite-austenite composite phase) is 10% in area% Or less, fine carbonitride precipitates having an average size of 100 nm or less are present in an amount of 0.03% or more by weight, and an average grain size of the ferrite structure is 20 μm or less.

The steel is 90% or more (excluding 100%) ferrite; And at least one of the remaining 10% or less of pearlite, cementite and MA (martensite-austenite composite phase).

The pearlite, cementite, and MA (martensite-austenite composite phase) are greatly effective in improving the strength of steel sheet, in particular, tensile strength, but when excessively increased, their size increases or becomes elongated by rolling. In this case, impact toughness deteriorates at low temperatures, and in particular, acts as a path for generation and propagation of hydrogen organic cracks, thereby rapidly deteriorating hydrogen organic crack resistance. Therefore, the total fraction of at least one of the pearlite, cementite and MA (martensite-austenite composite phase) is preferably limited to 10% or less in area%.

The pearlite may not exist in a banded streucture but may be present in a finely dispersed form having a degree of orientation (Omega_12) value of 0.1 or less as defined in ASTM E1268. The finely dispersed pearlite is present between the fine ferrites.

The steel is manufactured by a steel manufacturing method including a normalizing heat treatment step, and the fraction of ferrite, pearlite, cementite and MA in the microstructure after the normalizing heat treatment of the steel increases the ferrite by 10% or less compared to before the normalizing heat treatment. Perlite may decrease by less than 10%, cementite may increase by 5%, and MA may decrease by 3%.

In the ferrite tissue, fine carbonitride precipitates having an average size of 100 nm or less are present at 0.03% by weight or more.

When the fine carbonitride precipitate content is less than 0.03% by weight, coarse austenite is produced during the normalization heat treatment, and coarse austenite is produced, and coarse pearlite is formed during subsequent cooling, and at the same time, the strength of the precipitate is increased and There is a fear that the hydrogen fixation effect is greatly reduced.

The average grain size of the ferrite tissue is 20 μm or less.

When the average grain size of the ferrite structure exceeds 20 µm, there is a fear that the strength and hydrogen organic crack resistance due to the structure refinement decrease.

The steel may have a tensile strength of 485 MPa or more, a shock absorption energy value of 120 J or more at −40 ° C., and a longitudinal crack ratio (CLR) of 5% or less upon HIC evaluation.

The steel may have a thickness of 6 ~ 133mm.

Hereinafter, a method for producing a high strength steel having excellent hydrogen organic crack resistance (HIC resistance) and low temperature impact toughness according to another preferred aspect of the present invention will be described.

According to another preferred aspect of the present invention, a method for producing a high strength steel having excellent hydrogen organic crack resistance (HIC resistance) and low temperature impact toughness is weight%, C: 0.02 to 0.12%, Si: 0.02 to 0.60%, and Mn: 0.6 to 1.6%, P: 0.020% or less (excluding 0%), S: 0.003% or less (excluding 0%), Sol.Al: 0.002 to 0.060%, Cu: 0.005 to 0.6%, Ni: 0.005 to 1.0 %, Cr: 0.005-0.5%, Mo: 0.01-0.3%, Ti: 0.001-0.10%, Nb: 0.001-0.06%, V: 0.001-0.10%, N: 0.001-0.006%, Ca: 0.0002-0.0060% , Slab containing residual Fe and other unavoidable impurities, (Ti + V + 0.5Nb + 0.5Mo) is 0.05% or more and (Ti + V + 0.5Nb + 0.5Mo) / C is 0.8 or more. Reheating the furnace;

Obtaining a hot rolled steel by controlling and rolling the slab reheated as described above under a condition that a rolling end temperature is 850 ° C. or more;

A controlled cooling step of cooling the hot rolled steel to a cooling end temperature of 550 to 750 ° C. and then air cooling the hot rolled steel; And

It comprises a normalized heat treatment step of maintaining the steel cooled as described above 1.3 * t [t is steel sheet thickness (mm)] + (5 to 60 minutes) at 850 ~ 950 ℃, then air-cooled.

Reheating stage

The slab having the above-mentioned composition is reheated at 1080-1300 ° C.

When the reheating temperature is lower than 1080 ℃, it becomes difficult to re-use carbides, etc. generated in the slab during the performance, if the reheating temperature exceeds 1300 ℃ austenite grain size becomes too coarse and then the tensile strength and low temperature toughness of the steel sheet Mechanical properties such as can be greatly reduced. In particular, since the added Ti, Nb, V, Mo, etc. should be reheated to a temperature that can be sufficiently re-solubilized, the slab reheating temperature is preferably limited to 1080 ~ 1300 ℃. The more preferable slab reheating temperature is 1150-1280 degreeC.

Hot rolled steel  Getting steps

The slab reheated as described above is controlled and rolled under the condition that the rolling end temperature is 850 ° C. or more to obtain a hot rolled steel.

When the reheated slab is rolled in general, the rolling is terminated at too high a temperature so that the grain refining effect is small, and when controlled rolling to a too low temperature, reused Nb is precipitated as carbonitride, which is produced at high temperature. The precipitates thus obtained are large in size, which greatly reduces the effect of inhibiting austenite crystal growth during normalization. In addition, the coarse composite inclusions produced during the refining process must accommodate deformation due to rolling as the strength of the steel sheet increases as the rolling temperature is lowered, thereby segmenting the small inclusions or extending the amorphous inclusions. Such stretched or segmented inclusions are a direct cause of the generation and propagation of hydrogen organic cracks. In view of these phenomena, the rolling end temperature is preferably limited to 850 ° C or higher. More preferable rolling end temperature is 850-1050 degreeC. The rolling end temperature may be Ar ₃ + 70 ℃ ~ Ar ₃ + 270 ℃.

The hot rolled steel may have a thickness of 6 ~ 133mm.

Control cooling stage

The hot rolled steel is water cooled to a cooling end temperature of 550 to 750 ° C., followed by air cooling.

Control cooling for water cooling and air cooling is performed for the control rolled steel as described above. When the water is cooled after the control rolling, most of the ferrite is prevented from being generated at a high temperature during the water cooling, and the effect of producing water at a lower temperature than when air-cooled is great. In particular, when cooling is terminated in the range of 550-750 degreeC, fine carbonitride is produced | generated simultaneously with formation of a ferrite. The precipitate produced at this time is very fine, 100 nm or less. If the cooling end temperature exceeds 750 ° C, ferrite is produced and coarse at a high temperature, and precipitates are also coarse, which greatly reduces the effect. On the other hand, if the cooling end temperature is too low to be less than 550 ℃, a fine precipitate is not produced, the sufficient strength is not secured, or if the cooling rate is fast, ferrite production is suppressed and hard bainite phases are generated, but the strength is high, The problem is that the toughness at low temperatures is greatly reduced. Therefore, it is preferable to limit the cooling end temperature of controlled cooling to 550-750 degreeC in consideration of a component, rolling conditions, etc. More preferable cooling end temperature is 580-670 degreeC.

The steel after the controlled cooling has a fine structure of 90% or more (except 100%) of ferrite, and having a microstructure in which at least one total fraction of the pearlite, cementite and MA phases is 10% or less, and having an average size of 100 nm or less in the ferrite mouth. Precipitates may be present at least 0.03% by weight.

Normalizing  Heat treatment step

The steel cooled as described above is maintained at 1.3 * t [t is the thickness of the steel sheet (mm)] + (5 to 60 minutes) at 850 ~ 950 ℃, then air-cooled normalizing heat treatment.

If the holding temperature is lower than 850 ° C. during the normalizing heat treatment, it is difficult to re-use the solid solution solute elements, thereby making it difficult to secure the strength. On the other hand, if the holding temperature is higher than 950 ° C., grains grow to damage low-temperature toughness.

In addition, the reason for limiting the holding time during the normalizing heat treatment is that the homogenization of the tissue is difficult when the holding time is shorter than the reference time, and the productivity is impaired when the holding time is longer.

Therefore, the holding time during the normalizing heat treatment is preferably limited to 1.3 * t [t is the steel sheet thickness (mm)] + (5-60 minutes).

The steel microstructure after the normalizing heat treatment is ferrite; And at least one of the remaining pearlite, cementite and MA (martensite-austenite composite phase), wherein the fraction of ferrite is 90% or more (excluding 100%) in area%, and the pearlite, cementite and MA (martensitic) The total fraction of at least one species (site-austenite composite phase) is 10% or less by area%, and fine carbonitride precipitates of 100 nm or less are present in the ferrite tissue by weight% by 0.03% or more, and the average grain size of the ferrite tissue is present. The size may be 20 μm or less.

The fractions of ferrite, pearlite, cementite and MA in the steel microstructure after the normalizing heat treatment are increased by 10% or less, the pearlite is reduced by 10% or less, and the cementite is increased by 5% or less, compared to before the normalizing heat treatment. It can decrease below 3%.

As such, by optimally controlling the steel composition, controlled rolling, controlled cooling, and normalizing heat treatment methods according to the present invention, while minimizing hardened structures such as pearlite, cementite, and MA, which are more disadvantageous to toughness and hydrogen organic crack resistance than conventional methods, The refinement of the structure enables the production of high strength steels with high strength, low temperature toughness and resistance to hydrogen-organic cracking at the same time.

Tempering  Heat treatment step

The steel material subjected to the normalizing treatment may be temper-treated at 500 ° C. or higher for 10 minutes or more as necessary.

Tempering heat treatment may be carried out for the purpose of increasing toughness and strength at low temperatures. It is preferable to perform tempering temperature at 500 degreeC or more. If the tempering temperature is lower than 500 ° C., the removal of residual stress of the steel is not smooth or the generation of desired fine precipitates is insignificant. If the tempering heat treatment time is less than 10 minutes, since the formation of precipitates is insignificant, it is preferable to limit it to 10 minutes or more.

By the tempering heat treatment, the fraction of ferrite in the microstructure of the steel can be increased to area%, 3% or less, the total fraction of pearlite, cementite and MA (martensite-austenite composite phase) is area%, 3 It can decrease below%.

By the tempering heat treatment, the carbonitride precipitate content in the steel can be increased by 0.001 ~ 0.005% by weight.

Hereinafter, the present invention will be described in more detail with reference to Examples.

 (Example)

The slab having the steel composition (chemical composition) of Table 1 was reheated to the manufacturing conditions of Table 2, followed by control rolling of the reheated slab to obtain a hot rolled steel having a thickness of Table 2, followed by controlled cooling and normalizing. Heat treatment to prepare a steel.

At this time, the normalizing heat treatment time was 1.3 * t [t is steel sheet thickness (mm)] + 20 minutes.

In Table 1 below, the inventive materials (a to c) correspond to the steel composition of the present invention, and the comparative materials (d to h) are outside the steel composition of the present invention.

The fraction of ferrite (area%) and the average grain size (μm), the content of fine precipitates (wt%) and the average size (nm), the fraction of pearlite (area%), and the yield strength after normalizing the steels produced as described above. (MPa), tensile strength (MPa), impact absorption energy (J, -45 ℃), HIC CLR (%) was investigated and the results are shown in Table 3 and Table 4.

In Table 3, the microstructures other than ferrite and pearlite are at least one of cementite and MA (martensite-austenite composite phase).

On the other hand, for Inventive Example 1 and Comparative Example 3, the microstructure and the precipitate distribution were observed, and the results are shown in FIGS. 1 to 4. 1 shows a microstructure photograph of Comparative Example 3, FIG. 2 shows a microstructure photograph of Inventive Example 1, FIG. 3 shows a precipitate distribution photograph of Comparative Example 3, and FIG. 4 shows a precipitate distribution photograph of Inventive Example 1. FIG.

Ingredient
Chemical composition (% by weight) C Si Mn P S Sol.Al Cu Ni Cr Mo Ti Nb V N Ca Ti + V +
0.5 Nb +
0.5Mo (Ti + V +
0.5 Nb +
0.5Mo)
/ C foot
persons
ashes

a 0.055 0.32 1.14 0.003 0.0005 0.027 0.23 0.15 0.06 0.08 0.023 0.008 0.009 0.0032 0.0016 0.076 1.38 b 0.035 0.22 1.45 0.003 0.0008 0.031 0.01 0.14 0.02 0.12 0.043 0.002 0.002 0.0041 0.0012 0.106 3.03 c 0.081 0.43 0.91 0.005 0.0004 0.015 0.01 0.01 0.12 0.03 0.083 0.021 0.012 0.0029 0.0013 0.121 1.49 ratio
School
ashes



d 0.041 0.32 1.45 0.007 0.001 0.032 0.01 0.02 0.04 0.05 0.011 0.002 0.003 0.0042 0.0013 0.040 0.98 e 0.145 0.38 1.35 0.007 0.0009 0.022 0.12 0.33 0.02 0.11 0.012 0.003 0.002 0.0032 0.0021 0.071 0.49 f 0.089 0.33 0.82 0.006 0.0008 0.035 0.02 0.01 0.02 0.42 0.001 0.003 0.002 0.0045 0.0012 0.215 2.41 g 0.102 0.15 1.08 0.005 0.001 0.021 0.11 0.02 0.03 0.05 0.15 0.005 0.006 0.0034 0.0023 0.184 1.80 h 0.061 0.31 1.25 0.008 0.0009 0.035 0.01 0.03 0.02 0.09 0.021 0.003 0.003 0.0088 0.0018 0.071 1.16

Ingredient Thickness
(mm) Slab
Reheat
Temperature
(Degree) Rolling
Wrap-up
Temperature
(Degree) Cooling
End
Temperature
(Degree) Normalizing
Temperature
(Degree) Inventive Example 1 a 30.0 1230 880 635 891 Inventive Example 2 b 9.5 1190 862 712 903 Inventive Example 3 c 95.0 1163 982 623 888 Comparative Example 1 b 25.0 1062 885 642 916 Comparative Example 2 c 45.0 1163 936 513 913 Comparative Example 3 c 80.0 1134 981 - 905 Comparative Example 4 d 51.0 1223 963 635 899 Comparative Example 5 e 30.0 1198 924 621 913 Comparative Example 6 f 12.0 1178 872 602 900 Comparative Example 7 g 78.0 1192 974 655 897 Comparative Example 8 h 25.0 1153 875 638 887

Ingredient product
thickness
(mm) Ferrite fraction
(%) Fine precipitate content (%) Pearlite
Fraction (%) Ferrite Grain Average Diameter (㎛) Precipitation Diameter
(nm) Inventive Example 1 a 30.0 96.2 0.046 2.2 17 35 Inventive Example 2 b 9.5 98.9 0.065 0.7 13 48 Inventive Example 3 c 95.0 92.1 0.074 4.6 15 41 Comparative Example 1 b 25.0 98.9 0.022 0.9 23 55 Comparative Example 2 c 45.0 92.1 0.024 4.6 22 37 Comparative Example 3 c 80.0 88.2 0.015 8.5 31 120 Comparative Example 4 d 51.0 98.4 0.024 1.0 16 45 Comparative Example 5 e 30.0 83.7 0.043 9.6 25 113 Comparative Example 6 f 12.0 89.9 0.131 3.7 17 63 Comparative Example 7 g 78.0 88.2 0.112 6.9 26 145 Comparative Example 8 h 25.0 95.4 0.009 2.7 23 36

Ingredient Yield strength after normalizing
(MPa) After normalizing
Tensile Strength (MPa) Shock absorption energy after normalizing (J, -49 degrees) HIC CLR after normalizing
(%) Inventive Example 1 a 348 501 253 0.0 Inventive Example 2 b 380 515 314 0.0 Inventive Example 3 c 314 500 157 0.0 Comparative Example 1 b 319 447 301 0.0 Comparative Example 2 c 305 458 198 0.0 Comparative Example 3 c 281 412 67 52.0 Comparative Example 4 d 321 446 272 0.0 Comparative Example 5 e 354 522 49 12.0 Comparative Example 6 f 365 592 34 23.0 Comparative Example 7 g 337 517 61 7.0 Comparative Example 8 h 344 455 275 0.0

As shown in Table 3 and Table 4, the steel produced according to the steel composition and manufacturing conditions of the present invention (Invention Example 1-3) is a tensile strength of 485MPa or more, impact absorption energy value of 120J or more at -40 ℃ and HIC evaluation It can be seen that it has a longitudinal crack ratio (CLR) of less than 5%. Such steels of the present invention are very effective for use as steel for shipbuilding, marine and pressure vessels.

On the other hand, Comparative Example 1 is the component satisfies the scope of the invention, but the slab reheating temperature is lower than the scope of the invention, the coarse carbonitride precipitate produced during the slab production is not sufficiently reused to be produced as a fine precipitate during subsequent rolling, cooling The fraction is greatly reduced, resulting in little effect of strength increase after normalizing.

Although the component of Comparative Example 2 also satisfies the scope of the invention, in the cooling step after rolling, the cooling end temperature is lower than the range of the invention, so that the rate at which precipitates are produced becomes too slow and eventually sufficient carbonitride is not produced. In this case, too, the strength after normalizing is not sufficiently secured.

In Comparative Example 3, the component satisfies the scope of the invention, but in the conventional manufacturing method of air cooling without applying controlled cooling after rolling, ferrite transformation is started at a high temperature due to too slow cooling rate to produce coarse ferrite. In addition, the formation of fine precipitates is not enough. In this case, the final microstructure is not only coarse with ferrite (white part in the picture) as shown in FIG. 1, but also pearlite (black part in the picture) including a few MA tissues is formed in a band shape, so that strength and impact Both toughness and HIC properties are not sufficiently secured.

Comparative Example 4 shows a case where the added carbonitride generating element is not sufficient. Even if all the manufacturing conditions were manufactured in the scope of the invention, the amount of fine precipitates produced was not sufficient, and the strength was not sufficiently secured.

Comparative Example 5 shows a case where the amount of C added exceeds the scope of the present invention. As the content of C increases, the amount of precipitated carbonitrides increases, but when the ratio of C is too high than the alloying elements, the resulting carbonitrides are produced coarsely, and the ability to inhibit austenite growth during subsequent normalization decreases. In addition, the effect of increasing strength itself is also greatly reduced, resulting in the deterioration of strength and toughness at low temperatures.

Comparative Example 6 is a case where the added amount of Mo exceeds the scope of the present invention. Mo is an important element for making fine precipitates, while the solid solution, Mo, greatly increases the hardenability of the steel, encouraging residual austenite to form a martensite-austenite complex (MA constituent) during cooling after normalization. As a result, the yield strength of the steel is lowered, the tensile strength is greatly increased, and the toughness at low temperature is greatly reduced.

The comparative example 7 is a case where the addition amount of Ti exceeds the range of this invention. Ti is also the most important element to make fine precipitates, but when added excessively, precipitates are produced in very coarse size, which acts as a starting point for destruction, which greatly degrades toughness and hydrogen-organic crack resistance at low temperatures. .

Comparative Example 8 is a case where the amount of N added exceeds the scope of the present invention, when N is excessively added, even if the reheating temperature is increased, the inventory capacity of the coarse carbonitride produced during slab production is greatly reduced. Therefore, even if the following production conditions satisfy the scope of the present invention, the fraction of fine carbonitrides produced is significantly lowered, and thus a sufficient strengthening effect cannot be obtained.

On the other hand, the steel produced according to the conventional method (Comparative Example 3), as shown in Figure 1, while the pearlite is present in a banded (banded structure), the steel produced according to the present invention (Inventive Example 1) is shown in FIG. As shown in the present invention, not only the pearlite is present in finely dispersed form but in the form of a band, but also manufactured according to the present invention (Inventive Example 1) (FIG. 4) is a steel produced according to the conventional method (Comparative Example 3 It can be seen that the finer precipitates are distributed more than in FIG. 3).

As described above, in the case of manufacturing the steel according to the steel composition and manufacturing conditions of the present invention, high strength, low temperature toughness through the miniaturization of the structure while minimizing the hardened structure such as pearlite, which is more disadvantageous to toughness and hydrogen cracking resistance than conventional methods And it was confirmed that the hydrogen organic crack resistance can be obtained at the same time excellent effect.

Claims (11)

By weight%, C: 0.02 to 0.12%, Si: 0.02 to 0.60%, Mn: 0.6 to 1.6%, P: 0.020% or less (excluding 0%), S: 0.003% or less (excluding 0%), Sol Al: 0.002-0.060%, Cu: 0.005-0.6%, Ni: 0.005-1.0%, Cr: 0.005-0.5%, Mo: 0.01-0.3%, Ti: 0.001-0.10%, Nb: 0.001-0.06%, V: 0.001-0.10%, N: 0.001-0.006%, Ca: 0.0002-0.0060%, balance Fe and other unavoidable impurities, (Ti + V + 0.5Nb + 0.5Mo) is 0.05% or more, and (Ti + V + 0.5Nb + 0.5Mo) / C is 0.8 or more, ferrite as a microstructure; and one or more of the remaining pearlite, cementite and MA (martensite-austenite composite phase), and the fraction of the ferrite is 90% or more (excluding 100%) in area%, and the total fraction of at least one of the pearlite, cementite and MA (martensite-austenite complex phase) is 10% or less in area%, and has an average size of 100 nm in the ferrite tissue. The following fine carbonitride precipitates are present in an amount of 0.03% or more by weight, and The average grain size is 20 µm or less, and the pearlite has hydrogen organic crack resistance and low temperature impact toughness in a finely dispersed form having an Omega 12 value of 0.1 or less. Excellent high strength steels.
delete The hydrogen-induced crack resistance of claim 1, wherein the steel has a tensile strength of at least 485 MPa, a shock absorption energy value of at least 120 J at −40 ° C., and a longitudinal crack ratio (CLR) of 5% or less upon HIC evaluation. High strength steel with excellent low temperature impact toughness.
The high-strength steel of claim 1, wherein the steel has a thickness of 6 to 133 mm.
By weight%, C: 0.02-0.12%, Si: 0.02-0.6%, Mn: 0.6-1.6%, Sol.Al: 0.002-0.060%, Nb: 0.001-0.06%, V: 0.001-0.10%, Ti: 0.001 to 0.10% or less, Cu: 0.005 to 0.6%, Ni: 0.005 to 1.0%, Cr: 0.005 to 0.5%, Mo: 0.01 to 0.3%, Ca: 0.0002 to 0.0060%, N: 0.001 to 0.006%, P: 0.020% or less (excluding 0%), S: 0.003% or less (excluding 0%), balance Fe and other unavoidable impurities, (Ti + V + 0.5Nb + 0.5Mo) is 0.05% or more, ( Reheating the slab having Ti + V + 0.5Nb + 0.5Mo) / C of 0.8 or more to 1080-1300 ° C .;
Hot-rolling the slab reheated as above to obtain a hot rolled steel sheet such that the rolling end temperature is 850 ° C. or more;
Cooling the hot rolled steel sheet to a cooling end temperature of 550 to 750 ° C. by water cooling; And
Heat-treating the steel sheet cooled as described above at 850˜950 ° C. for 1.3 * t [t is steel sheet thickness (mm)] + (5 to 60 minutes); Hydrogen organic crack resistance and low temperature toughness excellent production method comprising a.
The method of manufacturing a high strength steel sheet having excellent hydrogen organic crack resistance and low temperature toughness according to claim 5, wherein the reheating temperature is 1150 to 1300 ° C in the reheating of the slab.
The method of manufacturing a high strength steel sheet having excellent hydrogen organic crack resistance and low temperature toughness according to claim 5, wherein the rolling end temperature is 850 to 1050 ° C.
The method of claim 5, wherein the rolling finish temperature is Ar ₃ + 70 ° C to Ar ₃ + 270 ° C.
The method of manufacturing a high strength steel sheet having excellent hydrogen organic crack resistance and low temperature toughness according to claim 5, wherein the hot rolled steel has a thickness of 6 to 133 mm.
The method of manufacturing a high strength steel sheet having excellent hydrogen-organic crack resistance and low temperature toughness, further comprising the step of tempering heat-treating the steel heat-treated according to the normalizing heat treatment at 500 ° C. or more for 10 minutes or more.
The method of manufacturing a high strength steel sheet having excellent hydrogen organic crack resistance and low temperature toughness according to claim 10, wherein the tempering heat treatment increases the content of carbonitride precipitates in the steel by 0.001 to 0.005 wt%.

Images